The placental isoenzyme of alkaline phosphatase is a cell surface protein frequently reexpressed in human tumors, as compared with normal tissues. We propose to develop a battery of monoclonal antibodies to placental alkaline phosphatase which will be used to develop improved immunoassays for its detection in serum and ultimately as probes to study the structure of the enzyme and its phenotypic variants that occur in pregnancy and cancer. The binding of the monoclonal antibodies to cultured tumor cells that express this enzyme will be tested for this uptake. Antibodies that display this characteristic will be examined by studying human tumors growing in nude mice. These studies will also provide improved reagents for the use of placental alkaline phosphatase as a tumor marker and, consequently, should also provide information on the feasibility of using antibodies to alkaline phosphatase for immunolocalization of tumors.